Google Maps GPS problems

I have an HTC Desire and I was using the GPS Google Maps at home and it seemed to work fine off GPS alone, I tried it on both WiFi and GPS separately. But when I left home and was near Niagara Falls the GPS would not lock and the maps would not fill in. I updated to Google Maps 4.5.1 and still no luck. Does anyone know why this would happen. Does Google maps only work on WiFi?

I was using strictly GPS no data connection what so ever. Also your last comment. But if the maps work when not on WiFi do you need to have the maps pre-viewed so they can load from the internal memory or SD card, because last I head Google Maps does not install maps to your mobile device, it works more like a PC and loads on demand.